As more experience is gained with the use of ultrasonic techniques for diagnostic purposes, there is a need to improve the quantification of the data obtained from these instruments. In particular, the ability to study discrete areas within the body with range-gated Doppler systems now offers the potential to obtain data on ventricular size and performance as well as imaging other cardiac geometries. However, a serious deficiency exists in the inability to accurately locate the sample-volume of interest and the direction of the sound beam within the body. While the slant-range from the transducer to the sample-volume is available, there is presently no adequate, real-time method of determining its position with respect to either an external or patient reference system. A microprocessor based system has been developed and evaluated which is capable of locating points along the sound beam to within plus or minus 0.5 mm. The system is non-constraining and permits freedom of movement during examination. A further instrument development phase is proposed to refine the instrument for clinical use. Procedures will also be developed to determine cardiac dimensions and volumes as well as image valvular jets.